2.2 What Determines Long-Term Temperature?

Many greenhouse gases emitted by human activities have long atmospheric residence times and alter the Earth’s energy balance. In addition, the average temperature of the Earth typically adjusts only slowly to changes in the energy balance (Lowe et al. 2009, Solomon et al. 2009). These slow-change processes imply that decision makers need to take into account long-term effects of current and near term emissions (National Research Council 2009). This is even more important as many impacts of climate change are potentially adverse and/or irreversible (at least on time scales of relevance to society).

A number of recent studies have shown that one of the strongest predictors of temperature increase within the twenty-first century is the cumulative emissions of greenhouse gases24, especially CO2 (Allen et al. 2009, IPCC 2007b, Matthews and Caldeira 2008, Matthews et al. 2009, Meinshausen et al. 2009, Van Vuuren et al. 2008). Cumulative emissions are determined by the annual emissions over time. In ambitious mitigation scenarios, the following factors play an important role in determining the cumulative emissions:

the year in which global emissions peak

the emissions level at the peak

the pathway of global annual emissions after the peak.

For the same cumulative emissions, a higher and/or later emissions peak means faster reductions after the peak than for earlier and/or lower peaks in emissions.

However, all three factors are bounded by feasibility considerations, including economic and/or technological constraints (see Section 2.3). For instance, there are constraints on how fast high-carbon energy infrastructure can be replaced with low-carbon infrastructure (for example, coal-fired power plants with renewable energy production).

As a consequence, there is a limited range of 2020 emissions that are consistent with a 2° C or 1.5° C limit, given current assumptions about the feasibility of emission pathways post 2020.

In addition, the probability of exceeding a particular temperature level varies according to the cumulative emissions level—for a higher degree of confidence in staying within a particular temperature limit, a lower cumulative emissions level is required. Pathways with later or higher peaks also reduce, or even eliminate, the “margin of error”, should future advances in climate science or additional evidence of the risks of climate change convince citizens and policymakers that more ambitious targets for limiting climate change are needed (Lowe et al. 2009).